Magnetic field sensing devices are widely used for contactless measurements of position and/or displacement in a broad range of technical fields, such as automotive applications, manufacturing, agricultural machines and consumer devices. Many of such measurement systems are based on detecting the position of a movable element along a measurement path by sensing the magnetic field created by the movable element, such as a permanent magnet, using magnetic field sensors distributed along the measurement path. For instance, magnetic sensors are frequently employed for measuring linear displacement of pistons in pneumatic and hydraulic cylinders and for level measurements of fluids in containers, such as in industrial ink tanks, diesel exhaust fluid (“DEF”) tanks, and fuel tanks. In general, the sensing signal output by each magnetic sensor varies with the strength of the applied magnetic field according to a known characteristic curve. Because the strength of the magnetic field applied to each magnetic sensor depends on the distance between the movable magnet and the respective magnetic sensor, the relative position of the movable magnet can be determined by analyzing the amplitude of the sensing signals acquired by a chain of sensors along the measurement path.
U.S. Patent Application No. 2005/0189938 A1, for example, describes a system and method for measuring the position of a movable magnet, in which the output from a chain of sensors (e.g., Hall effect sensors) is curve fit using an algorithm having a characteristic bell curve to determine an absolute and/or relative position associated with the magnet.
International Patent Application No. WO 2015/165593 A1 describes a similar example of a method and device for measuring the level of a liquid in a container in which the position of a movable float is determined by detecting the magnetic field created by the float with a line of magnetic field-sensitive sensor elements. The magnetic field generated by the float extends along the sensor line, which is substantially parallel to the measurement path, and is more intense at the sensors that are situated closest to the float. As a result, the signals acquired along the sensor line form a signal profile whose amplitude depends on the relative position of the float along the measurement path.
U.S. Pat. No. 9,297,634 B2 describes a device for generating a sensor signal, the profile of the sensor signal depending on the position of a magnetic field-generating element relative to the device. In this case, the sensor signal is obtained using at least two magnetically sensitive sensors disposed along a measurement path and a support field device. The support field device generates a magnetic support field in the magnetically sensitive sensors that has at least in the magnetically sensitive sensors an essentially identical direction and homogeneous field strength. The sensor signals can be represented as a table, which assigns a measurement value for the present position of the movable magnet to each sensor. Because the position of the sensors along the linear measurement path is already known, the table-like presentation corresponds to a representation of the signal progression in which the progression of measurement values is represented along a linear X-coordinate (the measurement path). The position of the element, at which it is located when the progression of the sensor signal is generated, is thereby determined by comparing the acquired progression with a stored reference progression.
The magneto-resistive magnetic sensors employed in many of the conventional techniques have a field response that depends on the applied magnetic field but may also depend on a temperature of the sensor. If the actual sensor temperature is not known and accounted for, the effect of temperature on the sensing signal may reduce the precision of the position and/or displacement measurement. Further, when detecting the position or displacement of a movable magnet using a chain of magnetic sensors that are affected by a temperature gradient along the chain, the precision of the measurement could be reduced since the distribution of temperature over the sensors will be reflected on the amplitude of the respective sensing signals and distort the signal profile obtained from the sensed magnetic field distribution.
Temperature gradients are inherent to main applications, such as fluid tanks. The effect of temperature gradients in the accuracy of position measurements performed using a chain of magnetic sensors might be particularly relevant for fluid level measurement devices, since the temperature of the fluid well below the liquid surface might be significantly different from the temperature at the liquid surface. In addition, since quite often the temperature distribution along the fluid depth is not known, it is assumed that the chain of sensors is at a homogeneous temperature, thereby affecting the precision of the level measurement.
In order to take into account temperature gradient effects, a temperature sensor could be provided close to each magnetic sensor for measuring the actual temperature of the respective magnetic sensor. However, this implies an increase in the volume of the measurement device as well as in production costs. Therefore, there is a need for a cost-effective solution that allows improving the accuracy of position and/or displacement measurements performed with magnetic sensing devices, and in particular, for magnetic sensing devices suitable for level or position measurements in environments where the establishment of temperature gradients can be expected.